Pierre-Emerick Aubameyang's spot kick secured a valuable win for Arsenal against top four rivals Manchester United.
The Gabonese striker bravely grabbed the ball after Fred took out Alexandre Lacazette following his miss vs Tottenham last week.
Granit Xhaka had put the hosts in the lead with a swerving shot that David De Gea bizarrely misjudged.
Ole Gunnar Solskjaer frantically tried to change things in search of a way back into the game.
But after Aubameyang's penalty, there was to be no famous comeback as the Norwegian suffered his first defeat as interim boss. Here are five things we learned.
1. Arsenal show true potential
It was only six weeks ago that Arsenal were embarrassed here by Manchester United in the FA Cup, picked apart on the break losing 3-1. That day it felt as if Arsenal had no real idea what they were doing, as the blindly played straight into United’s hands. Today was very different. Arsenal had a far clearer identity, far better tactics, and looked better prepared for the contest.
They did not throw too many men forward in attack, and the back line kept a far better handle on things than you might have feared. Their 3-4-1-2 system allowed them to dominate down the flanks and control midfield from the start, forcing United into a change.
It has been one step forward one step back for Arsenal all season but this certainly felt like progress, one of their best wins of the season, containing promise for next season too. The challenge now is to maintain it.
2. Aubameyang and Lacazette thrive together
There is a lot to be said for playing your best players. That might sound trite but it has not always been the case at Arsenal this season. Pierre Emerick Aubameyang and Alexandre Lacazette are obviously their best two players but Unai Emery has not always wanted to play the two together.
He did it consistently in January but after a 3-1 defeat at the Etihad Stadium he had a rethink, prompting a seven-game run in which it was always one or the other but never both. Here he returned to the strike pairing and was emphatically rewarded. Lacazette and Aubameyang’s movement was electric, never letting United relax, pulling their defence apart.
And Lacazette won the penalty that Aubameyang converted. Arsenal fans will hope for more of the same from one of the best old-fashioned strike pairings in the country.
3. Leno shows value
Fresh from his miraculous double save against Tottenham at Wembley last Saturday, Bernd Leno had another one of those games in goal when he looks like the best decision Arsenal have made in recent years. He looks more assured every week and shone again here under some serious examination.
He has uncanny reflexes and is quick off his line, as he showed here more than once darting out to stop Romelu Lukaku and Marcus Rashford. He even at one point ran 30 yards out from his goal to pass the ball, looking just as comfortable as Ederson does when he plays like that.
The fact that you could never picture Petr Cech doing that shows what an important change it was to replace him with Leno, helping Arsenal to play the way they want to.
4. Solskjaer shows proactive approach
There is more to Ole Gunnar Solskjaer’s management than just nostalgia and motivation. On Wednesday night in Paris Solskjaer hauled off Eric Bailly in the first half because United were struggling so much, a change that helped to turn the game their way.
Here he showed the same proactivity, making a tactical change long before half-time. United started with a 4-4-2 but it did not work, they were picked around in midfield and outnumbered down the flanks.
So Solskjaer switched to a 5-3-2 that gave them more stability in the middle and more cover out wide. It might not have been the obvious solution given the players he had, it meant Ashley Young playing as a makeshift centre-back. Ultimately United still lost, but this decision did at least give them the opportunity to get back into the game.
5. Ozil fails to convince
It says a lot about Mesut Ozil’s declining status at Arsenal that it was a surprise to see him here starting such a serious game. Because recently Unai Emery has only used him in low-key fixtures to unpick defensive teams.
In the last four months he has only started against Wolves, Qarabag, Burnley, Brighton, Cardiff City, BATE Borisov, Bournemouth and Rennes. And yet here he was up against Manchester United, given a platform to play in one of the biggest games of all.
But did Ozil do enough to persuade Emery that he should be the man to build this team around? Probably not.